Conventionally, hot-melt adhesives have been widely used to bond various adherends for their solvent-free nature and excellence in environmental resistance. Also, various types of photoreactive hot-melt adhesive compositions have been recently proposed which are applied by melt coating and then cured by irradiation. These types of hot-melt adhesive compositions are caused to cure by irradiation to finally exhibit high bond strength.
As one group of such compositions, a variety of photoreactive hot-melt adhesive compositions has been proposed which utilizes a ring-opening reaction of an epoxy resin.
For example, Japanese Patent Laying-Open No. Hei 11-5964 discloses a photoreactive hot-melt adhesive composition which contains an epoxy resin, a thermoplastic resin and a cationic photoinitiator.
Japanese Patent Laying-Open No. Hei 11-116929 discloses a photoreactive hot-melt adhesive composition which contains an epoxy compound as a cationically polymerizable compound, a cationic photoinitiator, and a stabilizer composed of a compound having a sulfur atom and a phenol group in a molecule.
Japanese Patent Laying-Open No. 2000-8015 discloses a photoreactive hot-melt adhesive which contains an epoxy compound having an alicyclic epoxy group and an epoxy group other than the alicyclic epoxy group, a compound having a phenolic OH group, and a cationic photo initiator. The epoxy compound has, on average, two or more of the structure represented by the following formula (6):
                (in the formula (6), m and n are independently 0, 1 or 2; and R15, R16, R17 and R18 independently represent a methyl, ethyl, isopropyl, isoamyl or phenyl group or a hydrogen atom).        
These photoreactive hot-melt adhesives are all designed to develop high bond strength through a ring-opening polymerization of an epoxy group. Also, selection in type of the epoxy compound or inclusion of a phenol-containing compound has been attempted to achieve improvements in bond strength or heat resistance after cure. Adherends are bonded together by these photoreactive hot-melt adhesives which are subsequently cured. However, when the bonded laminate after completion of curing is immersed in water, separation likely occurs at adherend-adhesive interfaces, which has been a problem.
In Japanese Patent Laying-Open No. Sho 63-248825, a curable composition is disclosed which comprises an epoxy resin, a UV catalyst and an interlocking agent containing a poly(alkylene oxide) residue portion. This reference describes that the composition, because of its tendency to show retarded curing after exposure to a UV radiation, can serve as an adhesive or bond opaque substrates during an open time after the exposure, for example. However, no description is provided in this prior art as to the water resistance or the like of the bonded laminate.
In Japanese Patent Laying-Open No. Hei 6-306346, an epoxy/polyester based hot-melt composition is disclosed which contains polyester that assumes a solid form at ordinary temperature, an epoxy resin, and a cationic photoinitiator. Due to the incorporation of the polyester that assumes a solid form at ordinary temperature, the adhesive reveals high strength immediately after lamination. This is described to eliminate the need of a clamping or other provisional fixing operation before curing completes. However, in the case where the composition excludes the solid polyester and incorporates the solid epoxy resin and the photoinitiator or where the composition incorporates the solid epoxy resin and the photoinitiator and further a low molecular weight hydroxyl-containing material, the adhesive according to this prior reference is described to reveal low strength immediately after lamination. Also, this prior reference lacks description as to water resistance or the like of the bonded laminate.
In general, it is not required that a cationically photoreactive hot-melt adhesive be irradiated continually until it is fully cured. That is, once such an adhesive is irradiated, curing proceeds by a dark reaction in which radiation-generated cationic active species participate. Accordingly, a cationically photoreactive hot-melt adhesive is often used where adherends once laminated by an adhesive prevent passage of a radiation through them and thus prevent exposure of the adhesive to a radiation, e.g., where opaque adherends are laminated together.
Although curing proceeds substantially by a dark reaction, a large number of cationic active species is generated in an exposed surface layer portion of the adhesive. This increases a tendency of the adhesive to form a low-adhesion cured film at its surface. The presence of the cured film makes it difficult for the adhesive to adhere surely to an adherend, particularly when an irradiation energy is excessively large or when the contact of the irradiated adhesive with an adherend is delayed. That is, the cationically photoreactive hot-melt adhesive does not show a substantial pot life before it is irradiated, but shows a relatively short pot life once it is irradiated.
In order to extend a pot life, a component called a cure retarder is generally incorporated in the cationically photoreactive adhesive. For example, although not a hot-melt adhesive, a liquid-form UV-curable resin composition is disclosed in Japanese Patent Laying-Open No. Sho 63-248825. This composition contains an epoxy resin, a UV-activated cationic catalyst and a compound having a polyalkylene oxide skeleton as a cure retarder. However, due to the absence of a hot-melt nature, this composition can not be coated into a thick film and its initial bond strength is low.
As described earlier, in Japanese Patent Laying-Open No. Hei 6-306346, a hot-melt composition is disclosed which contains a polyester resin that assumes a solid form at ordinary temperature, an epoxy resin, and a cationic photoinitiator. This reference describes a polyol component as being a preferred cure retarder.
However, such a cationic photoreactive adhesive cures substantially by a dark reaction. It accordingly takes a long time before curing thereof completes. In particular, the inclusion of a cure retarder slows down a rate of a cure reaction that proceeds by a dark reaction, while effective to extend a pot life. This creates a problem that the adhesive must be aged for a long time until its bond strength reaches a maximum.
Also, the adhesive while being cured does not exhibit high flexural strength. The adhesive does not develop sufficient flexural strength until it completes curing. This requires a long wait. Another problem arises where a cure retarder is incorporated in the adhesive. In such a case, complete curing thereof does not result in obtaining high flexural strength, while the reason is not clear.